The Galactic Center is an excellent laboratory for studying phenomena and physical processes that may be occurring in many other galactic nuclei. The Center of our Milky Way is by far the closest galactic nucleus, and observations with exquisite resolution and sensitivity cover 18 orders of magnitude in energy of electromagnetic radiation. Theoretical simulations have become increasingly more powerful in explaining these measurements. This review summarizes the recent progress in observational and theoretical work on the central parsec, with a strong emphasis on the current empirical evidence for a central massive black hole and on the processes in the surrounding dense nuclear star cluster. We present the current evidence, from the analysis of the orbits of more than two dozen stars and from the measurements of the size and motion of the central compact radio source, Sgr A*, that this radio source must be a massive black hole of about 4.4 \times 1e6 Msun, beyond any reasonable doubt. We report what is known about the structure and evolution of the dense nuclear star cluster surrounding this black hole, including the astounding fact that stars have been forming in the vicinity of Sgr A* recently, apparently with a top-heavy stellar mass function. We discuss a dense concentration of fainter stars centered in the immediate vicinity of the massive black hole, three of which have orbital peri-bothroi of less than one light day. This 'S-star cluster' appears to consist mainly of young early-type stars, in contrast to the predicted properties of an equilibrium 'stellar cusp' around a black hole. This constitutes a remarkable and presently not fully understood 'paradox of youth'. We also summarize what is known about the emission properties of the accreting gas onto Sgr A* and how this emission is beginning to delineate the physical properties in the hot accretion zone around the event horizon.

The galactic center massive black hole and nuclear star cluster

Mass loss is a key process in the evolution of massive stars, and must be understood quantitatively if it is to be successfully included in broader astrophysical applications such as galactic and cosmic evolution and ionization. In this review, we discuss various aspects of radiation driven mass loss, both from the theoretical and the observational side. We focus on developments in the past decade, concentrating on the winds from OB-stars, with some excursions to the winds from Luminous Blue Variables (including super-Eddington, continuum-driven winds), winds from Wolf–Rayet stars, A-supergiants and Central Stars of Planetary Nebulae. After recapitulating the 1-D, stationary standard model of line-driven winds, extensions accounting for rotation and magnetic fields are discussed. Stationary wind models are presented that provide theoretical predictions for the mass-loss rates as a function of spectral type, metallicity, and the proximity to the Eddington limit. The relevance of the so-called bi-stability jump is outlined. We summarize diagnostical methods to infer wind properties from observations, and compare the results from corresponding campaigns (including the VLT-flames survey of massive stars) with theoretical predictions, featuring the mass loss-metallicity dependence. Subsequently, we concentrate on two urgent problems, weak winds and wind-clumping, that have been identified from various diagnostics and that challenge our present understanding of radiation driven winds. We discuss the problems of “measuring” mass-loss rates from weak winds and the potential of the NIR Br
 α
 -line as a tool to enable a more precise quantification, and comment on physical explanations for mass-loss rates that are much lower than predicted by the standard model. Wind-clumping, conventionally interpreted as the consequence of a strong instability inherent to radiative line-driving, has severe implications for the interpretation of observational diagnostics, since derived mass-loss rates are usually overestimated when clumping is present but ignored in the analyses. Depending on the specific diagnostics, such overestimates can amount to factors of 2 to 10, and we describe ongoing attempts to allow for more uniform results. We point out that independent arguments from stellar evolution favor a moderate reduction of present-day mass-loss rates. We also consider larger scale wind structure, interpreted in terms of co-rotating interacting regions, and complete this review with a discussion of recent progress on the X-ray line emission from massive stars. Such emission is thought to originate both from magnetically confined winds and from non-magnetic winds, in the latter case related to the line-driven instability and/or clump-clump collisions. We highlight as to how far the analysis of such X-ray line emission can give further clues regarding an adequate description of wind clumping.

http://www.usm.uni-muenchen.de/people/puls/papers/review_mdot.pdf

Mass loss from hot massive stars

Context. Nuclear star clusters (NSCs) are located at the photometric and dynamical centers of the majority of galaxies. They are among the densest star clusters in the Universe. The NSC in the Milky Way is the only object of this class that can be resolved into individual stars. The massive black hole Sagittarius A* is located at the dynamical center of the Milky Way NSC.Aims. In this work we examine the proper motions of stars out to distances of 1.0 pc from Sgr A*. The aim is to examine the velocity structure of the MW NSC and acquire a reliable estimate of the stellar mass in the central parsec of the MW NSC, in addition to the well-known black hole mass.Methods. We use multi-epoch adaptive optics assisted near-infrared observations of the central parsec of the Galaxy obtained with NACO/CONICA at the ESO VLT. Stellar positions are measured via PSF fitting in the individual images and transformed into a common reference frame via suitable sets of reference stars.Results. We measured the proper motions of more than 6000 stars within ~1.0 pc of Sagittarius A*. The full data set is provided in this work. We largely exclude the known early-type stars with their peculiar dynamical properties from the dynamical analysis. The cluster is found to rotate parallel to Galactic rotation, while the velocity dispersion appears isotropic (or anisotropy may be masked by the cluster rotation). The Keplerian fall-off of the velocity dispersion due to the point mass of Sgr A* is clearly detectable only at R 0.3 pc. Nonparametric isotropic and anisotropic Jeans models are applied to the data. They imply a best-fit black hole mass of 106  . Although this value is slightly lower than the current canonical value of 4.0  106  , this is the first time that a proper motion analysis provides a mass for Sagittarius A* that is consistent with the mass inferred from orbits of individual stars. The point mass of Sagittarius A* is not sufficient to explain the velocity data. In addition to the black hole, the models require the presence of an extended mass of 106 in the central parsec. This is the first time that the extended mass of the nuclear star cluster is unambiguously detected. The influence of the extended mass on the gravitational potential becomes notable at distances 0.4 pc from Sgr A*. Constraints on the distribution of this extended mass are weak. The extended mass can be explained well by the mass of the stars that make up the cluster.

/pdf/the-nuclear-star-cluster-of-the-milky-way-proper-motions-and-3a73tvp8fg.pdf

The nuclear star cluster of the Milky Way: proper motions and mass

Nuclear star clusters (NSCs) are located at the photometric and dynamical centers of the majority of galaxies. They are among the densest star clusters in the Universe. The NSC in the Milky Way is the only object of this class that can be resolved into individual stars. We measured the proper motions of more than 6000 stars within ~1.0 pc of the supermassive black hole Sgr A*. The full data set is provided in this work. We largely exclude the known early-type stars with their peculiar dynamical properties from the dynamical analysis. The cluster is found to rotate parallel to Galactic rotation, while the velocity dispersion appears isotropic (or anisotropy may be masked by the cluster rotation). The Keplerian fall-off of the velocity dispersion due to the point mass of Sgr A* is clearly detectable only at R ~0.4 pc from Sgr A*. Constraints on the distribution of this extended mass are weak. The extended mass can be explained well by the mass of the stars that make up the cluster.

Context The GC is the closest galactic nucleus, offering the unique possibility of studying the population of a dense stellar cluster surrounding an SMBHAims The goals of this work are to develop a new method of separating early and late type stellar components of a dense stellar cluster based on narrow band filters, applying it to the central parsec of the GC, and conducting a population analysis of this areaMethods We use AO assisted observations obtained at the ESO VLT in the NIR H-band and 7 intermediate bands covering the NIR K -band A comparison of the resulting SEDs with a blackbody of variable extinction then allows us to determine the presence and strength of a CO absorption feature to distinguish between early and late type starsResults This new method is suitable for classifying K giants (and later), as well as B2 main sequence (and earlier) stars that are brighter than 155 mag in the K band in the central parsec Compared to previous spectroscopic investigations that are limited to 13–14 mag, this represents a major improvement in the depth of the observations and reduces the needed observation time Extremely red objects and foreground sources can also be reliably removed from the sample Comparison to sources of known classification indicates that the method has an accuracy of better than ~87% We classify 312 stars as early type candidates out of a sample of 5914 sources Several results, such as the shape of the KLF and the spatial distribution of both early and late type stars, confirm and extend previous works The distribution of the early type stars can be fitted with a steep power law ( = -149 ±  012), alternatively with a broken power law, = -108 ± 012, = -346 ± 058, since we find a drop in the early type density at ~10″ We also detect early type candidates outside of 05 pc in significant numbers for the first time The late type density function shows an inversion in the inner 6″, with a power-law slope of = 017 ± 009 The late type KLF has a power-law slope of 030 ± 001, closely resembling the KLF obtained for the bulge of the Milky Way The early type KLF has a much flatter slope of (014 ± 002) Our results agree best with an in-situ star formation scenario

/pdf/composition-of-the-galactic-center-star-cluster-population-2um22xrhbm.pdf

Composition of the galactic center star cluster - Population analysis from adaptive optics narrow band spectral energy distributions

Using adaptive optics at the Gemini North telescope, we have obtained a K-band spectrum of the star near the center of the luminous Galactic center bow shock IRS 8, as well as a spectrum of the bow shock itself. The stellar spectrum contains emission and absorption lines characteristic of an O5-O6 giant or supergiant. The wind from such a star is fully capable of producing the observed bow shock. However, both the early spectral type and the apparently young age of the star, if it is single, mark it as unique among hot stars within 1 pc of the center.

https://iopscience.iop.org/article/10.1086/507764/pdf

The K-Band Spectrum of the Hot Star in IRS 8: An Outsider in the Galactic Center?

High-resolution images in the H and K bands obtained by the Gemini North Telescope of the peculiar Galactic center source, IRS 8, reveal a central pointlike object enveloped in a remarkable bow shock, whose apex is located 02 to the northeast. The H-K' color of the bow shock is considerably redder than that of the central star. A United Kingdom Infrared Telescope (UKIRT) K-band spectrum reveals that the combined spectrum of the point source and bow shock is nearly featureless and that no shocked line emission (e.g., from H2) is physically associated with the bow. We interpret the bow as resulting from the interaction of the envelope or wind of the central star of IRS 8 with the extension of the northern arm of Sgr A West and/or the circumnuclear disk, and its emission as coming from radiatively heated, and possibly shock heated, dust.

A bow shock of heated dust surrounding galactic center source IRS 8

A Bow Shock of Heated Dust Surrounding the Galactic Center Source IRS 8

High resolution images in the H and K′ bands obtained by the Gemini North Telescope of the peculiar Galactic center source, IRS 8, reveal a central pointlike object enveloped in a remarkable bowshock, whose apex is located 0.2″ to the northeast. The H − K′ color of the bow shock is considerably redder than that of the central star. A UKIRT K band spectrum reveals that the combined spectrum of the point source and bow shock is nearly featureless and that no shocked line emission (e.g., from H2) is physically associated with the bow. We interpret the bow as resulting from the interaction of the envelope or wind of the central star of IRS 8 with the extension of the northern arm of Sgr A West and/or the molecular ring, and its emission as coming from radiatively- and possibly shock-heated dust.

A Bow Shock of Heated Dust Surrounding IRS 8

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The K-Band Spectrum of the Hot Star in IRS 8: An Outsider in the Galactic Center?

A bow shock of heated dust surrounding galactic center source IRS 8

A Bow Shock of Heated Dust Surrounding the Galactic Center Source IRS 8

A Bow Shock of Heated Dust Surrounding IRS 8